High temperature diffusion furnaces are well known to the semiconductor industry. Heat treatment in high temperature diffusion furnaces is one of the many steps in the manufacturing process of silicon wafers. Typically, process gas is injected into the heat treatment process in order to alter the composition of the wafers. For example, heat treatment of wafers with an appropriate process gas allows doping elements such as boron to be introduced into the molecular structure of the semiconductor material.
A high temperature diffusion furnace may include multiple furnace modules capable of heat treating multiple sets of silicon wafers. The individual diffusion furnace modules comprise a heating element which includes a process chamber for a process tube which is shielded by a liner. The process chamber has a load end where the wafers are inserted and a source end where process gas is injected. In addition, scavenger boxes are included to remove excess process gas along with an energy kit. Typically, the furnace modules are housed in a stacked orientation.
The heating elements are generally cylindrical in shape and symmetrical. The heating elements have an outer metallic housing, usually comprised of stainless steel or aluminum, and inner layers of insulating materials such as ceramic fiber. A furnace chamber is created between heating coils and a heating element liner. Several heating coils are secured together to form a continuous coil, with the middle heating coil enabling optimal temperature at the middle of process tube. The end heating coil is operated to enable a temperature in the process tube sufficient to overcome losses out the end of the furnace and to preheat any gasses being introduced into the furnace. The heating coil is generally a helical resistance wire made of chrome-aluminum-iron alloy, the wires generally heavy gauge (0.289" to 0.375" in diameter) for extended heating coil life at an elevated temperature.
The maximum permissible operating temperature for the heating coil alloy is 1400.degree. C. Since a temperature differential exists between the heating coil and the inside of the process tube, diffusion furnaces are normally operated at a maximum operating process tube temperature of 1300.degree. C.
Silicon wafers are heat treated in the middle section of the process tube. The process tube is fabricated of quartz, polysilicon, silicon carbide or ceramic. The process tube in sheathed by a liner which separates the heating coils from the process tube creating a furnace chamber. The silicon wafers to be heat treated are mounted onto boats and loaded either manually or automatically into the center of the process tube from the load end of furnace. The boats used to hold the wafers are generally fabricated of quartz, polysilicon, silicon carbide or ceramic.
Multiple furnace module configurations have increased heat treatment process capability and flexibility. Separate heat treatment processes at various stages may be performed in separate process tubes simultaneously. However, a system incorporating a multiple furnace module configuration increases the likelihood that a process in one furnace module may be affected by processes in other furnace modules. Moreover, a failure of a single furnace module may effect the ability to process wafers in other furnace modules. These limitations of multiple furnace module designs, as well as heating element design limitations which affect the performance of the heat treatment process, are as follows:
Furnace Material:
Typically, prior art furnaces are constructed with painted steel frames with aluminum or steel panels. While the materials used for the construction of the furnace have suitable strength, weight and thermal conductance characteristics, the surfaces are relatively irregular and non-uniform.
The semiconductor manufacturing process is performed in low particle environments or clean rooms. Furnaces may be placed in these clean rooms or adjacent grey rooms. Grey rooms require a less stringent atmospheric particle standard than clean rooms, yet are relatively particle free.
The materials used to construct equipment housed in a clean or grey room should have surfaces as uniform as possible with minimum surface irregularities. Typically, furnaces are positioned in grey rooms with the process tube accessible from the clean room. Therefore, the exterior surface of the furnace should be compatible with a clean room environment while consistent with furnace material requirements.
Furnace Leveling:
Prior art furnaces use individual leveling mounts to level the furnace. Generally, the individual level mounts are positioned at the bottom corner area of the furnace. Each individual mount may be adjusted to lower or raise the corresponding corner of the furnace in order to level the furnace.
The use of individual leveling mounts in a high temperature diffusion furnace has introduced three problems. First, when the furnace is moved to another location or first installed, the individual leveling mounts are susceptible to breakage. If the furnace is slid across the floor, excessive side loading may be applied to the individual leveling mounts causing deformation or breakage. Second, the individual leveling mounts create an open area under the furnace where debris may collect which is difficult to access for cleaning. Finally, the individual leveling mounts may restrict the weight distribution of the furnace and may not meet seismic requirements. Therefore, while the individual leveling mounts allow for positioning of the furnace, they introduce multiple disadvantages in the prior art furnace design.
Furnace Module Access:
Access to the individual furnace modules for repair and maintenance is generally difficult and time consuming. Furnace tube modules are normally accessed at the load end or source end of the process chamber requiring extensive disassembly of the furnace. Multiple disconnections and removal of components are necessary in order to remove the heating element. For example, all power connections are with nut and bolt type connections which need to be removed. Furthermore, these connections have a tendency to loosen over a period of time and require retightening to prevent overheating from oxidation due to poor electrical contact.
Access to the furnace module is further complicated by the size and weight of the heating element. A heating element can weigh as much as 200 lbs. This complicated removal of the heating element requires a significant amount of manhours to repair or maintain an individual heating element, which adds significantly to the overall cost of operating the furnace. In addition, safety of the maintenance personal is at risk because there is no mechanism for removing and positioning the heating element.
Furnace Heating Element/Process Chamber Interface:
The heat treatment of the wafers may be compromised by interaction of process gas with the furnace heating element. Typically, process gas is injected into the process tube from the source end while the furnace is placed in a grey room with the load end accessible from a clean room. Clean rooms reduce introduction of unwanted particles by creating a pressure differential between the clean room and adjacent rooms by maintaining a higher pressure in the clean room. This pressure differential between the load end and the rest of the furnace element can cause air currents to flow through the gaps around the process tube and the vestibule blocks, into the furnace chamber and finally out the gaps at the source end of the furnace. Process gas byproducts exiting the process tube at the load end may be carried into the furnace chamber by these air currents where they can react with the heating element wire and shorten the life of the heating element. The temperature non-uniformity introduced by the turbulence in the furnace chamber then may affect the uniformity of the heat treatment of the wafers.
The process gas may also react with the heating coils to produce unwanted by-products which may react undesirably with the processing of the semiconductor wafers.
In addition to the undesirable reaction of process gas in the furnace chamber, the heating element may also directly corrupt the wafers. Prior art heating elements use heavy silicon carbide liners to provide protection against heavy metal migration from the heating element materials of construction through the quartz process tube. Migration of the heavy metal may damage the silicon wafers being processed within the tube. The heavy liners are constructed totally of silicon carbide and are approximately 1/4 inch thick (6-7 mm). The weight of the silicon carbide significantly slows the response characteristics of the heating of the process chamber.
Furnace Cooling:
Residual heat and cooling from furnace modules may also introduce non-uniformities into the wafer fabrication process. For example, one furnace module may be operating at a steady state temperature while an adjacent furnace module may be heating up to a high temperature. The increase in energy loss from the shell of a furnace module which is heating up may change the ambient temperature of the adjacent furnace modules which are in steady state. This requires the control system to respond by making adjustments to maintain the desired temperature inside the process tube. However, the adjustments in the control system contribute to temperature instability.
In the prior art, multiple furnace module configurations are cooled by a single cooling system. Generally, a single forced air flow is generated by fans placed at the top of the furnace. A single flow of forced air is directed from all of the furnace modules and eventually out an exhaust at the top of the furnace. This single cooling system for all the furnace modules eliminates the capability of running heat treatment processes in the other furnace modules when repairs or maintenance to the cooling system are necessary.
Thermocouple Positioning and Composition:
In order to enhance the performance of the heat treatment process, the furnace chamber must be measured precisely within a fraction of a degree at very high temperatures in order to control the temperature of the furnace chamber. However, the composition and position of the thermocouples used to measure the furnace chamber have limited precision.
Present thermocouples are inserted into the furnace chamber through the side of the heating element, perpendicular to the center line axis of the process tube. This positioning has limited the thermocouple's ability to read actual temperatures in the furnace chamber because the thermocouple is not sufficiently immersed into the heating chamber. In order to obtain accurate measurements, the thermocouple should be sufficiently immersed into the furnace chamber at least six to eight times the diameter of the thermocouple to overcome heat loss or heat sink effect of the thermocouple leads and supporting ceramic sheath. For example, a 3/16 inch diameter thermocouple requires at least one and 1/8 inch to one and 1/2 inch of immersion into the furnace chamber in order to properly read actual furnace chamber temperature.
The perpendicular placement of the thermocouple also increases the likelihood of obtaining inaccurate temperature readings when the process tube is removed from the furnace module. In prior art systems, when the process tube needs to be removed for maintenance or repair, it is necessary for the thermocouples to be withdrawn from their operating position so that the removal of the process tube does not damage the thermocouples. When the process tube is reinserted, the repositioning of the thermocouples to their original position is very difficult. Therefore, the heating element must be re-profiled to obtain the required uniformity within the heating element. High current pulses required by the heating element also introduces inaccuracies into the thermocouple's measurement of furnace temperatures. The helically coiled, heavy-gauge wire used in the heating element requires high current for operation. However, this high current pulsed on-and-off for tight control by zero-crossover, silicon controlled rectifiers causes radio frequency interference (RFI) to be induced onto the thermocouple measuring circuit. The induced RFI causes errors in the control signal which make it difficult to control all the heating element within tight temperature tolerances, such as .+-.0.10.degree. C as required by many of todays semiconductor processes.